The increasing demand for life extension of both military and civilian
aircrafts leaded to significant advances in repair technology of cracked
metallic structures. Thus, composite patch repair of metallic structures
became a rapidly grown technology in the field of aerospace. The eddy current
method is used to trace crack propagation under a composite patch repair
of a cracked metallic structure, after mechanical testing in fatigue. The
capability and the reliability of the eddy-current method to detect cracks
under a composite obstacle of significant thickness are checked for several
patch thicknesses. Notched specimens 6mm thick were fabricated using 2024-T3
Aluminum. Boron Epoxy patches bonded with film adhesive were applied to
the one side of the metallic specimens. Initial notches were 10mm long,
while the thickness of the reinforcement was varying from 2 layers (0.25mm)
to 7 layers (0.875mm) in order to represent actual structural composite
patch repairs. Crack propagation from the tip of the notches was achieved
by fatigue loads. The estimation of required loads to cause fatigue crack
propagation was done by means of three-dimensional finite elements analysis.
The eddy current method was then applied to trace the crack tip under the
patch after their mechanical testing. Accuracy of the eddy-current method
was verified by measuring the crack lengths on both sides of the specimen
and comparing the results. The eddy-current method was found to be fully
capable of tracing the crack propagation under the composite patch, requiring
only proper calibration for the generator. Small differences in the crack
lengths between the patched and the unpatched side of the specimen were
explained by their non-symmetric configuration, which induced different
stress intensity factors at the patched and the unpatched sides, as finite
elements analysis has clearly shown.
Key words: Composite Patch Repair, Eddy-Current, Fatigue,
Non Destructive Inspection, Composite Materials.